An impact power tool is designed to perform a tightening task by applying a rotational striking impact to an output shaft (anvil) with a hammer rotated by a motor output power. Since the impact power tool is operable at a high speed and with an increased torque, it has been extensively used in construction sites, fabrication factories and so forth. Japanese Patent Laid-open Publication No. H5-200677 discloses an impact power tool in which a shutoff function for automatically stopping operation of the tool upon reaching a desired torque is realized by counting the number of strikes and then determining whether the number of strikes thus counted has reached a value corresponding to a desired tightening torque.
The maximum tightening force of the impact power tool is decided by the rotational speed of the hammer (i.e., the rotating speed of a motor) which in turn depends on the voltage of a driving power source. For that reason, if the shutoff is performed by using only the number of strikes, it may become impossible to manage the tightening torque of bolts and nuts in a process of a factory requiring such management. This is because the tightening torque is gradually reduced as the voltage of a battery used as a driving power source undergoes gradual reduction as a result of continuous operation.